This invention relates to a magnetic sensor for detecting a weak magnetic field as a stray magnetic field generated in an automobile, a ship, an aircraft, a domestic electrical appliance, a medical instrument, and the like.
A conventional magnetic sensor comprises a toroidal inductor accommodated in a bobbin having at least one detection coil wound thereon. The toroidal inductor comprises a ring-shaped or toroidal core contained in a ring-shaped core case on which a toroidal coil as an excitation coil is wound.
The magnetic sensor is a transformer. When an exciting AC voltage as a primary voltage is applied to the excitation coil, AC magnetic fluxes are induced to flow in the toroidal core. Therefore, a secondary voltage as an output voltage is induced on the detection coil. When the stray magnetic field is applied to the toroidal core, the AC magnetic fluxes flowing in the toroidal core are affected by the stray magnetic field and change. As a result, the output voltage is changed in the amplitude. That is, the output voltage is amplitude-modulated by the stray magnetic field. Therefore, the stray magnetic field can be detected from the amplitude variation of the output voltage.
Manufacture of the above-mentioned magnetic sensor requires troublesome winding operations for forming the excitation coil and at least one detection coil. Particularly, in the winding operation for the excitation coil, a wire must be wound on the toroidal core, actually the ring-shaped core case to form the toroidal excitation coil. This winding operation is quite troublesome because the wire is made to repeatedly pass through an inner bore of the ring-shaped core case until it is wound around the core case by the number of turns ranging from several tens to several hundreds. Thus, it takes a long time for such a winding operation.
If the toroidal inductor has a small size, the inner bore of the ring-shaped core case has a small diameter. In this event, it becomes difficult to perform the winding operation for making the wire pass through the inner bore of the ring-shaped core case. The result is that miniaturization of the toroidal inductor is limited.
Recently, there are available in commerce small-sized magnetic sensors using printed circuit boards stacked holding a relatively thin toroidal core therebetween. The printed circuit boards have, as thin conductor patterns, sections of excitation coils and detection coils. Those sections are connected to one another through through-holes in the circuit boards to thereby form those coils.
Although the known magnetic sensor of the printed circuit type is formed in a small size, it is low in reliability. This is because that the thin toroidal core is readily deformed and/or destroyed by mechanical strain of printed circuit boards caused by undesired external force applied to the printed circuit boards.
In the printed type magnetic sensor, it is impossible to use a bulk type toroidal core which has a relatively large axial size. Further, the toroidal core is badly affected by heat during the reflowing operation to solder the conductor patterns of the printed circuit boards.